Conventional techniques relating to manganese oxide having a layer structure include the technique described in patent literature 1 below. The manganese oxide disclosed therein is a manganese dioxide nanobelt having a layer structure. Patent literature 1 describes the manganese dioxide nanobelt as being capable of intercalating and deintercalating cations with ease. The manganese dioxide nanobelt is produced by wet synthesis using an aqueous alkali solution, more specifically by heating dimanganese trioxide powder in an aqueous sodium hydroxide solution at a temperature ranging from 150° to 200° C. for 72 hours or longer.
There is another known method for producing an oxide of manganese by wet synthesis using an aqueous alkali solution, which includes the steps of adding an ammoniacal alkaline solution, a hydrogen peroxide solution, or the like to a solution containing manganese ions to form a precipitate of manganese hydroxide, separating the precipitate from the mother liquid, dispersing the separated precipitate in an organic solvent, and drying and grinding the precipitate at 120° C. or lower (see patent literature 2 below).
Apart from these techniques, patent literature 3 (see below) discloses a manganese dioxide crystal having a rock salt type crystal structure belonging to the cubic system (space group Fm-3 m). The manganese dioxide crystal is described as being useful as a high-capacity electrode material with a higher volumetric energy density than manganese dioxide having a conventional crystal structure.
Apart from the manganese dioxide described in patent literature 3, non-patent literature 1 below reports manganese dioxide having a hexagonal crystal structure with an a-axis length of 11.08 Å and a c-axis length of 4.45 Å.